Hydrogen Abstraction by the Nighttime Atmospheric Detergent NO3·: Fundamental Principles

The nitrate radical (NO3·) was first identified as early as the 1881, but its role in atmospheric oxidation has only been identified within recent decades. Due to its high one-electron reduction potential and its reactivity toward a diverse set of substrates, it dominates nighttime atmospheric oxidation and has since been the subject of much work. Despite this, studies on NO3· hydrogen atom transfer reactions have been somewhat neglected in favor of its more reactive oxidative pathways.

The first section of the dissertation will highlight the role of substrate structure, solvent effects, and the presence of a polar transition state on NO3· hydrogen abstractions from alcohols, alkanes, and ethers. In this work the acquisition of absolute rate constants from previously unexamined substrates was analyzed alongside a curated list of common organic pollutants degraded through hydrogen atom abstraction. It was found that NO3· reacts with low selectivity through an early polarized transition state with a modest degree of charge transfer. Compared to the gas-phase, condensed-phase reactions experience rate enhancement—consistent with Kirkwood theory—as a result of the polarized transition state.

These insights are then applied to abstractions by NO3· from carboxylic acids in the next section. It was found that the rate constants for abstraction of α-carbons were diminished through induction by the adjacent carbonyl compared to the activation seen for the aforementioned substrates. The deactivation of abstraction by the carbonyl was found to be dramatically reduced as the substrate's alkyl chain was lengthened and/or branched. This apparent change in mechanism coincides with hydrogen abstraction of the alkyl chain for sufficiently large carboxylic acids and rules out the possibility of concerted bond breaking elsewhere in the molecule.

Finally, the dissertation will cover some additional projects related to the overall nature of the work including examination of the kinetics of radical clock systems when complexed with metal ions and the examination of a highly oxidative biosourced monomer.